Hydrocarbon gaseous fuel is conventionally used for fuel cells. This fuel is normally oxygen free, but may contain varying amounts of sulfur. In order to use the fuel within the fuel cell, the sulfur must be removed. The fuel is also reformed to increase the molecular hydrogen content of the fuel before using it in the fuel cell.
The gas supply is usually natural gas. During peak demand conditions on the fuel delivery system, additional gas such as propane is supplied in the fuel. Since this would increase the heating value, air is added to maintain the heating value at its normal level.
For most systems using natural gas, this peak shaved natural gas creates no problem. The oxygen content does, however, create a problem for the fuel treatment system for fuel cells. It must therefore be removed with the oxygen being consumed in a catalyzed reaction with hydrogen.
U.S. Pat. No. 4,181,503 illustrates most of the basic components of a prior art system. A portion of the reformed fuel is recycled and mixed with in-coming cold fuel to provide an ample molecular hydrogen concentration for subsequent reactions in the processing train. This fresh fuel along with the recycled portion is then passed to an oxidizer where the oxygen is consumed by reacting with the hydrogen in the presence of a catalyst. A minimum temperature is required for ignition, this temperature depending on the particular catalyst and whether or not the catalyst has been sulfided because of sulfur in the fuel. Once the reaction starts, any sulfur on the catalyst will be burned off.
If there is no oxygen present in the fuel, the temperature leaving the oxidizer will be the same as that entering. If oxygen is present, however, the exothermic reaction causes a temperature increase. This temperature increase is about 500.degree. F. (260.degree. C.) with 4 percent oxygen content. The temperature leaving the oxidizer is therefore a variable depending on the oxygen content of the fuel at any particular time.
From the oxidizer the fuel then enters the hydrodesulfurizer which converts sulfur in the fuel to H.sub.2 S. This hydrodesulfurizer requires a minimum temperature of 500.degree. F. (260.degree. C.) for the reaction, but at temperatures exceeding 650.degree. F. (343.degree. C.) the reaction reverses and accordingly these high temperatures cannot be tolerated.
Accordingly, in addition to the preheater, one or two heat exchangers are required between the oxidizer and the hydrodesulfurizer. Where the temperature entering the oxidizer is less than 500.degree. F. (260.degree. C.) with no oxygen present in the fuel, the temperature to the hydrodesulfurizer would be less than 500.degree. F. (260.degree. C.). Accordingly, a heat exchanger would be required to add heat to the fuel.
On the other hand, a 4 percent oxygen content will produce a 500.degree. F. (260.degree. C.) temperature increase in the oxidizer. Even operating at the upper limit of the hydrodesulfurizer this means that the maximum allowable temperature entering the oxidizer would be 150.degree. F. (65.degree. C.). This temperature cannot be depended on to initiate the oxidation reaction.
Continuing through the fuel treatment train an H.sub.2 S removal apparatus removes the H.sub.2 S from the gas.
Steam is added to the gas at a temperature level such as to produce greater than 600.degree. F. (326.degree. C.) gas steam mixture for entrance into the reform reactor. This steam is controlled to maintain a constant ratio with the fuel passing therethrough in accordance with the known system. The fuel and steam are fed into the reform reactor where, in the presence of a catalyst, additional hydrogen is formed. The exit temperature from the regenerative reform reactor is on the order of 700.degree. F. (371.degree. C.).
Heat is removed from the fuel stream to reduce the temperature to approximately 350.degree. F. (176.degree. C.) where the fuel enters a shift converter which increases the molecular hydrogen content of the fuel stream. The converted fuel exits at 450.degree. F. (232.degree. C.) with a portion being recycled and the remainder going to the fuel cells for use therein.
The prior art system where the entire fuel mixture is heated prior to entrance to the oxidizer requires multiple heat exchangers and complex operation.